In electrophotographic photoreceptors, photoconduction takes place through (1) a process of generating electric charge upon exposure to light and (2) a process of charge transport.
Electrophotographic photoreceptors wherein both the processes (1) and (2) are effected by the same substance include selenium photoreceptors, and those wherein the processes (1) and (2) are separately carried out by different substances include a combination of amorphous selenium and poly-N-vinylcarbazole. The latter technique in which the processes (1) and (2) are effected by different substances advantageously allows a wide choice in the kind of materials to be used for photoreceptors, which leads to improvement of electrophotographic characteristics, such as sensitivity of photoreceptors, acceptable potential, etc. Further, such a large choice of materials means that materials favorable to the formation of photoreceptor coating films can be selected from a wide range.
Photoreceptors in electrophotography are essentially required to satisfy the following performance properties: (1) they can be charged at an appropriate potential in the dark; (2) dark decay of the charge is small; (3) they are capable of rapidly discharging upon light irradiation; and the like. The above-described inorganic substances certainly have many merits but, at the same time, involve various demerits. For example, selenium, which is widely employed at this time, fully satisfies the above requirements (1) to (3), but involves difficult conditions of production, with the ultimate disadvantage of increased production cost. Further, it is difficult to shape selenium in the form of a belt due to its lack of flexibility, and selenium photoreceptors need delicate handling due to high sensitivity to heat and mechanical shocks. Photoreceptors in which cadmium sulfide or zinc oxide is dispersed in a resin binder have mechanical drawbacks in terms of surface smoothness, hardness, tensile strength, abrasion resistance, and the like, so that they cannot be used repeatedly.
In attempts to overcome these disadvantages of inorganic materials, electrophotographic photoreceptors using various organic materials have recently been proposed, and some of them have been turned to practical use. For example, U.S. Pat. No. 3,484,237 discloses a photoreceptor comprising poly-N-carbazole and 2,4,7-trinitrofluoren-9-one; Japanese Patent Publication No. 25658/73 discloses a photoreceptor comprising poly-N-vinylcarbazole sensitized with a pyrylium compound; and Japanese Patent Application (OPI) No. 10785/72 describes a photoreceptor comprising mainly an eutectic complex composed of a dye and a resin (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Another attempt has been directed to highly sensitive electrophotographic photoreceptors comprising a combination of a material capable of generating electric charge by light (called charge generating material) and a material capable of transporting the generated charge (called charge transporting material). Examples of such photoreceptors so far proposed include a photoreceptor having a charge generating layer having further provided thereon a charge transporting layer as disclosed in U.S. Pat. No. 3,791,826; a photoreceptor having a charge transporting layer having further provided thereon a charge generating layer as disclosed in U.S. Pat. No. 3,573,906; and a photoreceptor having a light-sensitive layer comprising a charge transporting material having dispersed therein a charge generating material as taught in U.S. Pat. No. 3,764,315. With respect to this type of photoreceptors, many useful charge generating materials have so far been proposed, but not so many as for truly useful chage transporting materials. Excellent charge transporting materials are those capable of transmitting light of a wavelength causing a charge generating material to generate charges through themselves to the charge generating material, maintaining a sufficient potential when charged, and rapidly transporting the charges generated by the charge generating material.